In the United States colorectal cancer (CRC) is the third leading cause of cancer mortality. Surgery is the primary treatment for early stage CRC; but adjuvant therapy is usually needed in advanced disease. Approximately 55% of CRC patients have metastatic disease for which the prognosis is poor despite incremental improvements due to introduction of irinotecan, oxaliplatin and targeted agents. The latter include the antibodies, cetuximab and panitumumab, initially approved by the FDA for patients who fail chemotherapy. The antibodies were selected for their ability to kill tumors by blockade of the epidermal growth factor receptor (EGFR). How- ever, although the anti-EGFR antibodies promised efficacy with reduced toxicity compared to chemotherapy, less than 12% of CRC patients respond to the antibodies as single agents. Resistance is largely due to mutations that activate the KRAS oncogene to drive CRC growth independently of EGFR signaling. The FDA has restricted cetuximab and panitumumab to treatment of metastatic CRC that expresses non-mutated KRAS. A new therapeutic approach in CRC might be to exploit an alternative mechanism of action for cetuximab called antibody-dependent cellular cytotoxicity (ADCC). Cetuximab can potentially engage innate killer cells (K cells) to kill tumor cells by ADCC, regardless of whether or not the tumor has the KRAS mutation. However, when cetuximab is used as a single agent, little clinical benefit appears to come from ADCC. One way to improve the impact of ADCC would be to combine cetuximab with an agent that can enhance the efficiency of tumor killing by K cells. ARI-4175 has been selected for this purpose because preliminary results suggest it can stimulate cetuximab-mediated ADCC to produce tumor responses in KRAS mutant CRC in mice. The Specific Aim is to demonstrate that ARI-4175 can amplify cetuximab-mediated ADCC to produce significant tumor responses in nude mice challenged with a human KRAS mutant CRC cell line (HCT-116) that is resistant to cetuximab as a single agent. Tumor responses to treatment will be measured, tumor attack by K cells will be assessed by histology and immunostaining of responsive tumors, and the effect of experimental therapy on ADCC will be assayed ex vivo. The approach will seek to substantiate preliminary evidence suggesting that ARI-4175 activates a particular type of K cell-the natural killer (NK) cell-to kill tumors by ADCC. For ARI-4175 to be a viable drug candidate it must interact with cetuximab to produce significant tumor responses in the HCT-116 model with evidence of an immune mechanism of action. If the STTR Phase I study is successful, IND-enabling studies and the initiation of a phase 1 clinical trial of ARI-4175 will be proposed for STTR Phase II.